Human proprotein convertase subtilisin/kexin type 9 (PCSK9) is a secreted protein expressed primarily in the kidneys, liver and intestines. It has three domains: an inhibitory pro-domain (amino acids 1-152; including a signal sequence at amino acids 1-30), a serine protease domain (or catalytic domain; at amino acids 153-448), and a C-terminal domain (or cysteine/histidine-rich domain) of 210 residues in length (at amino acids 449-692), which is rich in cysteine residues. PCSK9 is synthesized as a zymogen that undergoes autocatalytic cleavage between the pro-domain and catalytic domain in the endoplasmic reticulum. The pro-domain remains bound to the mature protein after cleavage, and the complex is secreted. The cysteine-rich domain may play a role analogous to the P-(processing) domains of other Furin/Kexin/Subtilisin-like serine proteases, which appear to be essential for folding and regulation of the activated protease.
PCSK9 is a member of the proteinase K secretory subtilisin-like subfamily of serine proteases (Naureckiene et al., 2003 Arc. Biochem. Biophys. 420:55-67) and functions as a strong negative regulator of hepatic low density lipoprotein receptors (LDL-R). PCSK9 plays a critical role in cholesterol metabolism by controlling the levels of low density lipoprotein (LDL) particles that circulate in the bloodstream. Elevated levels of PCSK9 have been shown to reduce LDL-R levels in the liver, resulting in high levels of low density lipoprotein cholesterol (LDL-c) in the plasma and increased susceptibility to coronary artery disease. (Peterson et a.l, J Lipid Res. 49(7): 1595-9 (2008)).
The low-density lipoprotein receptor (LDL-R) prevents atherosclerosis and hypercholesterolemia through the clearance of the low-density lipoproteins (LDL) in the bloodstream. LDL-R is regulated at the posttranslational level by PCSK9. PCSK9 knockout mice showed an approximate 50% reduction in the plasma LDL-c levels and showed enhanced sensitivity to statins in reducing plasma LDL-c (Rashid S, et al (2005) Proc Natl Acad Sci 102:5374-5379. Human genetic data also support the role of PCSK9 in homeostasis. Mutations in PCSK9 are associated with abnormal levels of LDL-c in the blood plasma (Horton et al., 2006 Trends. Biochem. Sci. 32(2):71-77). Two mutations were recently identified that are presumably “loss-of-function” mutations in PCSK9. The individuals with these mutations have an approximately 40% reduction in the plasma levels of LDL-c which translates into an approximate 50-90% decrease in coronary heart disease.
Therefore, it would be highly advantageous to produce an inhibitor of PCSK9 that antagonizes the activity of PCSK9 and blocks or reduces the corresponding role PCSK9 plays in various pathologic conditions.